In recent years, an oxide semiconductor has attracted attention as a novel semiconductor material having high mobility, which is an advantage of polysilicon, and a uniform element characteristic, which is an advantage of amorphous silicon.
In Patent Document 1, a field effect transistor which includes, as an oxide semiconductor, an oxide including indium (In), zinc (Zn), and gallium (Ga) (a material having an In—Ga—Zn—O composition) has been proposed.
[Reference]
    [Patent Document 1] Japanese Published Patent Application No. 2006-173580